C++ function not working in different class [closed] - c++

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I am really struggling with using functions and classes, and on VS, I get a "Matrix::Matrix()" is unaccessible error, and I just cannot figure out why.
I'm trying to learn about functions and classes and just am not getting it! The program basically uses a constructor and destructor in a class called Matrix, and creates a 'matrix'. I am now trying to use a function within this matrix class to get the value from inside of the constructed Matrix, and it will return the correct value, but don't understand why I am getting this error.
#include <iostream>
using namespace std;
class Matrix
{
Matrix();
private:
int M;
int N;
double *data;
int get(int i, int j){
return data[i*N+j];
}
//CONSTRUCTOR
public:
Matrix(int sizeR, int sizeC,double * input_data)
{
M=sizeR;//Rows
N=sizeC;//Columns
data = new double[M*N];//creation of 1D array, uses m&n values
cout<<"\nMatrix::Matrix(int sizeR, int sizeC, double * input_data) is invoked\n\n";
//ENTER DATA INTO MATRIX HERE:
for(int i=0;i<M*N;i++)//Loops for every data entry into 1D array, uses r&c as referenece to size
data[i] = input_data[i];//Accesses each value at specific location, inputs value 'val'
for(int i=0;i<M*N;i++)//Loops for every data entry into 1D array
cout<<data[i]<<" ";
}
//DECONSTRUCTOR
~Matrix(){
//delete data
delete []data;
cout<<"\n\nMatrix::~Matrix() is invoked\n\n";
}
};
int main()
{
int sizeR, sizeC, g1, g2;
g1 = 2;
g2 = 2;
Matrix M1;
cout<<"Enter No. Rows: ";
cin>>sizeR;
cout<<"Enter No. Columns: ";
cin>>sizeC;
double * input_data;
input_data = new double[sizeR*sizeC];
//INPUTS VALUES TO ARRAY
for(int i=0;i<sizeR*sizeC;i++)//Loops for every row
input_data[i] = i+1*input_data[i-1];
Matrix(sizeR, sizeC, input_data);
cout<<"Find value at row: ";
cin>>sizeR;
cout<<"Find value at column: ";
cin>>sizeC;
M1.get(g1, g2);
}

Answer 1

The default access modifier for classes in C++ is private so if you didn't explicitly used the public: access modifier before the definition of the constructor Matrix() then it would be considered to be private.

Answer 2

You want to create M1 object using the constructor you provided.
The line Matrix(sizeR, sizeC, input_data); doesn't create M1 object.
You need to do Matrix M1(sizeR, sizeC, input_data); or create the default constructor with public accessibility.
Furthermore (as somebody pointed out) M1.get(...) will also cause errors if Matrix::get() is not public.

Answer 3

Just place your default constructor (declaration/definition) in your class' public section:
class Matrix {
public: // <<<<
Matrix() {
}
Matrix(int sizeR, int sizeC,double * input_data) {
// ...
}
// ...
};
The default scope policy for class is private unless specified differently.

Answer 4

There is no definition of the constructor which takes no parameters, and further its declaration is private. Therefore, the call
Matrix M1;
fails. Just provide a definition in your class such as
Matrix() {}
and make that public accessible and it will work fine.
EDIT: The get function as well need to be public. Next, this raw array is a bad thing which leeds to a double-free corruption error. Use a std::vector instead. This becomes code-review, I guess. See here for a still quick-n-dirty but somewhat improved implementation.